The present invention relates to a fabrication method and tool for boring a hole in a hard and brittle material, particularly in a glass plate, and chamfering the aperture on the upper-side and both-sides of the plate.
A diamond drill is a tool for boring a hole by removing the workpiece material by grinding with a diamond wheel portion 12 attached to the end of a shank 11 of steel as shown in FIG. 5. In most cases a hollow space 13 penetrating along the center axis of the drill is provided.
In boring a hole in hard and brittle meterials, a diamond drill excells in fabrication efficiencies and fabrication accuracies but have a disadvantage in often chipping-off on the edge of the aperture of a hole it has bored. The chipping-off not only affects the accuracy and appearance but also can lead to a fracture of the glass plate by giving a starting point for a crack.
This chipping-off occurs on the side to which the drill cuts through, and therefore can be prevented by a method where the boring is started on both side of the plate. The boring from the one side stops at the half point of the plate thickness, and the other boring on the other side continues to arrive at the center of the plate thickness for making a through hole.
To make a chamfering, the upper-side and both-sides of the plate is respectively fabricated with a diamond wheel for chamfering after the through hole has been completed. As is shown in the FIG. 6 the chamfering can be made simultaneously with a boring operation with a drill provided with an taperd portion 24 on the upper side of grind wheel portion 22 with a specified diameter.
The above mentioned fabrication from both side of the plate will require two spindles placed opposite each other in alignment in a same axis and the machine used as well as its operation is much complicatd. The alignment between the spindles is not always correct, and therefore such a disadvantage tends to occur as an misalignment between the holes from both side at the point of penetration of the entire hole.
In addition, the apertures on both side of the plate have no chipping off, and however there still remains the risk to make a starting point of a crack in the penetration point within the plate. In order to prevent a method using drills with diameters somewhat different for both side of the plate is used, and however this gives a stepped hole or joggles.
The height H of the portion with a specified diameter in the diamond drill shown in FIG. 6, capable of chamfering is required to be appropriate to the thickness of the glass plate or the workpiece. To control H so as to meet the plate thickness to be fabricated or to compensate H for the wear of the drill tip, a structure is adopted that makes the height H to be adjustable by fitting a diamond wheel body 35 provided with a tapered portion 34 on the drill proper 32. The tip of the tapered portion 34 has a edge of an acute angle, and therefore is fast in its rate of wear, and the rounded tip configuration resulted is copied on the chamfered surface which can not keep uniformity any more. In other words, these chamfering method needs a tool of a complicated structure, and moreover does not give a good finished surface.